The use of conductive polymer particles in electronics applications is well known. Typically, metal coated polymer beads are positioned between component surfaces and serve to provide electrical connection between those components.
In EP-A-1135777, gold plated polymer microbeads are suggested for use in electrical components. Styrene core particles are palladinated to allow attachment of the gold coating to the particle surface. EP-A-265212 describes gold plated particles with anisotropic conductivity.
U.S. Pat. No. 5,866,202 describes the manufacture of metallised amino resin polymer particles, prepared by acid-catalysed polycondensation and plated using a mixture of electroless and electro plating techniques.
U.S. Pat. No. 6,906,427 relates to conductive particles and methods for their manufacture. The invention described therein relates to the use of various plating methods to obtain a polymer particle with multiple metal layers of low melting point metal. The nature of the polymer particles used is very broadly defined although styrene/divinyl benzene polymers are exemplified.
The present inventors sought to improve upon known conductive particles and have realised that polymer particles swollen with an aromatic compound/aldehyde polymer or urea/aldehyde polymer and which are subsequently optionally heat treated provide an ideal polymer particle for coating and hence for use in electrical components. The heat treated polymer particles have remarkable mechanical properties, especially in terms of one of more of higher elastic (Young's) modulus, lower thermal expansion coefficient, greater deformation before fracture and better thermal resistance. Moreover, the particles swollen with an aromatic compound/aldehyde polymer or urea/aldehyde polymer provide a much better surface for coating upon and hence prevent delamination of the coating from the particle.
A particle with improved properties such as compression resistance, and/or a reduced thermal expansion coefficient has significant applications in the electronics industry, for example in ball grid arrays (BGA), chip-scale packaging (CSP), and conductive adhesives and electronic components in general such as in circuit boards. The problems caused by failure of BGA packages due to fatigue stresses and or reliability issued are discussed, for example, in WO 97/001866 and in M. Loeffler, “Polymer-Core Solder Balls: An Alternative to Solid Solder Balls”, CircuiTree (2006) 23 May 2006.
To be used in such applications, particles need to be coated with a conductive material and there is a further need for improvements in the surface adhesion of the polymer core particles to any conductive material in order to prevent delamination and to ensure uniform plating and fewer surface defects in the final particles. The inventors have also realised that treatment of particles with amines before coating improves surface adhesion and provides a much better surface for coating.
The actual polymer particle onto which a coating is applied is not itself new. WO93/02112 describes the production of polymer particles in which a seed particle is, for example, treated with sulphuric acid and swollen with resorcinol and formaldehyde before undergoing a condensation polymerisation to produce a polymer particle.
Whilst therefore the polymer core particles according to the current invention can be prepared using the method of WO 93/02112, no one before has appreciated how valuable these polymers are in conductive particle technology. Moreover, the present inventors have found that heat treatment of these polymer particles surprisingly provides the improved mechanical properties so vital for a useful conductive particle. Finally, the present inventors have realised that surface treatment of the particles provides a surface ideal for coating upon.